The expression and control of bioregulatory macromolecules in embryo-fetal development and the effect of their interactions on cellular proliferation, differentiation and immunocompetence are being studied. Mouse placentas contain interferon-like factors (P1-IFN), mitogenic stimulating factors (MSF) and transforming growth factors (TGF). The P1-IFN exhibits species specific antiviral activity and shows serological cross reactivity to mouse fibroblast interferon, but not to mouse immune interferon. The antiviral activity is acid and trypsin sensitive and is blocked by actinomycin D. MSF and TGF, in contrast, do not exhibit species specificity and are moderately stable to acid treatment. Chromatography of high salt placental extracts by gel filtration resolves two major peaks of antiviral activity, one with an estimated molecular weight greater than 65 K and the other approximately 30 K, as well as a more diffuse peak (15 K - 30 K) exhibiting the major growth promoting activities (MSF and TGF). The two peaks of antiviral activity, which vary quantitatively during gestation, also display antiproliferative activity specific for mouse cells. Additionally, treatment of mouse cells with fibroblast interferon abrogates the MSF activity of placental extracts.